The present invention relates to a combustor for burning fuel in compressed air. More specifically, the present invention relates to a combustor for a gas turbine that significantly reduces the amount of NOx produced by combustion.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range in firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. Although it is known that combustion at lean fuel/air ratios reduces NOx formation, such lean mixtures are difficult to ignite and exhibit poor flame stability.
It is therefore desirable to provide a combustor capable of stable combustion with very lean mixtures of fuel and air, so a to reduce the formation of NOx, as well as one that permits reliable ignition at start-up.